1. Field of the Invention
The present invention relates to an inkjet recording apparatus which is capable of ejecting particulate matter such as pigment matter and toner matter by making use of an electric field, and more particularly to voltage control for the inkjet recording apparatus.
2. Description of the Related Art
There has recently been a growing interest in non-impact recording methods, because noise while recording is extremely small to such a degree that it can be neglected. Particularly, inkjet recording methods are extremely effective in that they are structurally simple and that they can perform high-speed recording directly onto ordinary medium. As one of the inkjet recording methods, there is an electrostatic inkjet recording method.
The electrostatic inkjet recording apparatus generally has an electrostatic inkjet recording head and a counter electrode which is disposed behind the recording medium to form an electric field between it and the recording head. The electrostatic inkjet recording head has an ink chamber which temporarily stores ink containing toner particles and a plurality of ejection electrodes formed near the end of the ink chamber and directed toward the counter electrode. The ink near the front end of the ejection electrode forms a concave meniscus due to its surface tension, and consequently, the ink is supplied to the front end of the ejection electrode. If positive voltage relative to the counter electrode is supplied to a certain ejection electrode of the head, then the particulate matter in ink will be moved toward the front end of that ejection electrode by the electric field generated between the ejection electrode and the counter electrode. When the coulomb force due to the electric field between the ejection electrode and the counter electrode considerably exceeds the surface tension of the ink liquid, the particulate matter reaching the front end of the ejection electrode is jetted toward the counter electrode as an agglomeration of particulate matter having a small quantity of liquid, and consequently, the jetted agglomeration adheres to the surface of the recording medium. Thus, by applying pulses of positive voltage to a desired ejection electrode, agglomerations of particulate matter are jetted in sequence from the front end of the ejection electrode, and printing is performed.
A first example of such an electrostatic inkjet recording apparatus has been disclosed in Japanese Patent Unexamined Publication No. 62-13379. According to this conventional apparatus, a pulse voltage of one of positive and negative polarities and a predetermined pulse width is applied to the counter electrode to achieve stable inkjet recording regardless of ink resistance.
A second example has been disclosed in Japanese Patent Unexamined Publication No. 1-204750. This conventional apparatus is provided with a bias means and a pressure generating mechanism. The bias means applies a constant bias voltage to a counter electrode. The pressure generating mechanism periodically presses an ink chamber to form constant meniscuses at the ejection nozzle. With the constant bias voltage applied, a recording pulse is further applied to a selected one of the recording electrode in synchronization with the periodical pressing operation of the pressure generating mechanism.
In the first conventional example (Publication NO, 62-13379), however, the pulse voltage is applied to the counter electrode so as to inject charges into ink meniscuses regardless of the recording pulse signal for ejecting ink from the recording electrode. In other words, the pulse voltage applied to the counter electrode is not designed for ink ejection.
On the other hand, the second conventional example (Publication NO. 1-204750) needs the pressure generating mechanism for periodically presses the ink chamber to form constant meniscuses at the ejection nozzle. Therefore, the structure of the inkjet head becomes complicated, resulting in increased cost. Further, the bias voltage is continuously applied to the counter electrode during inkjet recording operation.